1. Field of the Invention
This invention relates to television cameras.
2. Description of the Prior Art
Two examples of previously proposed television cameras will first be described with reference to FIGS. 1 and 2 respectively of the accompanying drawings, which are block diagrams of the cameras.
The television camera of FIG. 1 comprises a pick-up tube 1 forming a pick-up element (a solid-state pick-up element such as a charge coupled device (CCD) may also be utilized) and an automatic iris device 2 associated with a light receiving target of the pick-up tube 1. The automatic iris device 2 drives an iris thereof by a motor or a meter. A lens system (not shown) is also provided. The picked-up signal from the pick-up tube 1 is supplied, through a preamplifier 3 and a variable gain amplifier 4, to a video processor 5 which supplies an output picked-up video signal to an output terminal 6.
First and second comparators 7 and 8, which may be differential amplifiers, supply first and second compared output signals to the automatic iris device 2 and to the variable gain amplifier 4 respectively to control them.
First and second variable reference level (voltage) signal sources 9 and 10 supply first and second reference level (voltage) signals to noninverting input terminals of the first and second comparators 7 and 8, respectively. The picked-up signal from the variable gain amplifier 4 is supplied to a level detector 11 wherein a video level (voltage) detecting signal is detected and supplied to an inverting input terminal of the second comparator 8 so that the video level thereof is compared with the second reference voltage.
The picked-up signal from the preamplifier 3 is supplied to another level detector 11' wherein another video level (voltage) detecting signal is detected. The signal thus detected is supplied to an inverting input terminal of the first comparator 7 so as to compare the video level with the first reference voltage.
In the television camera of FIG. 2, the video level (voltage) detecting signal from the level detector 11 is also supplied to the inverting input terminal of the first comparator 7 thereby to compare the video level with the first reference voltage.
Thus, in the television cameras of FIGS. 1 and 2, there are respectively formed first and second automatic control loops (an automatic iris control loop and an automatic gain control (AGC) loop) 12 and 13 respectively to control the automatic iris device 2 and the variable gain amplifier 4.
These television cameras have certain drawbacks as follows. The television camera of FIG. 1 is provided with two level detectors 11 and 11', thus causing a complicated arrangement. Moreover, the total response time of the control loops 12 and 13 is the product of the response times of the first and second control loops 12 and 13, so that it becomes too long. Accordingly, the stabilization time of the automatic control loop becomes long.
The television camera of FIG. 2 does not have these defects, but instead, the automatic control tends to be unstable. Moreover, even when an adequate amount of light is incident on the pick-up tube 1, the light energy entering the pick-up tube 1 via the automatic iris device 2 does not assume an optimum value in practice, thus causing deterioration of the signal to noise (S/N) ratio of the picked-up signal.